The rising tide

• By Peter Zaborowsky
• Mar 01, 2000

Water recycling or reuse is expanding rapidly throughout the United States. There are more than 1,000 reuse systems in place in the United States. Most are in the South and Southwest, in areas that have limited water supplies. One of the oldest systems began operation in the 1960s at the Grand Canyon.

Even after 40 years, the terminology used in water reuse projects can still be confusing. Terms describing the use of treated wastewater include water reuse, successive use, reclamation and recycling. Although there have been attempts to distinguish between these terms, they are commonly used interchangeably. Recently, several communities such as the city of Aurora, Colo., in 1999, adopted recycled water as the term to describe their systems, since the public easily understands the term and recognizes the benefits of recycling.

Reuse benefits

Some of the early water reuse systems were designed as a means of wastewater disposal. Land application of wastewater was popular in the 1970s and was studied extensively as an effective means of treating municipal wastewater. These systems were based on the ability of vegetation to uptake nutrients and contaminants in the wastewater. Crops grown on land application sites were a secondary benefit of the systems.

In water-short areas, however, wastewater effluent has long been recognized as a valuable resource. In Colorado, for instance, water rights owners have depended upon effluent return flow to satisfy their water rights. With growing urban populations and the resulting water demand, the cost of water continues to increase. Thus, communities looked for ways to cost effectively satisfy their water demands view reuse as a viable alternative.

Types of reuse

There are several different types of water reuse. These vary from simple water accounting practices to highly complex potable water augmentation systems.

Agricultural exchanges. Agricultural exchanges are a common means of reusing water, where water owned by farmers is "exchanged" upstream and used by a municipality for drinking water. The water then passes through the city's potable water system and into the wastewater collection and treatment systems, is discharged and subsequently used for irrigation by the farmer. Agricultural exchanges often involve complex administrations of water rights and may even need approval from other water rights holders who may be affected.

Non-potable reuse (urban landscape irrigation). Reusing water for landscape irrigation is a common reuse configuration, particularly in arid or semi-arid areas. These systems use treated wastewater for the irrigation of turf and ornamental vegetation. The level of treatment depends upon the extent of public exposure. For unrestricted public access, such as a city park, an advanced treatment such as filtration is normally required. Non-potable reuse systems result in dual distribution piping to separately deliver potable and non-potable water to the points of use.

Industrial reuse. There is a wide variety of industrial water reuse systems throughout the United States. They include internal recycling of waste streams and the use of treated municipal wastewater for cooling and other industrial purposes. Depending upon the use, additional treatment may be required. For instance, cooling water may need to be softened to avoid scale formation in the cooling system.

Indirect potable reuse. Indirect potable water reuse occurs where reclaimed water is introduced into the raw water supply of a community. The raw water is subsequently treated and distributed to customers. Indirect potable reuse can be further classified as either planned or unplanned. Unplanned indirect potable reuse occurs when one community withdraws water from a stream, uses it for municipal purposes, then discharges treated wastewater into the drinking water supply of a downstream community that subsequently withdraws it for their potable water purposes. Planned indirect potable reuse systems have similar configurations, but include advanced treatment systems such as membrane or granular activated carbon systems, prior to augmenting the drinking water supply.

Direct potable reuse. Direct potable reuse systems introduce highly treated wastewater directly into the potable water distribution system. Although direct potable reuse has been studied extensively, there are no operating systems in the United States.

Potable water reuse is much more complex and costly than non-potable reuse due to the more rigorous water treatment and testing requirements as well as issues of public perception and acceptance. For most communities non-potable reuse is the most viable option to consider, especially for urban landscape irrigation.

There are, however, many technical and institutional issues that must be considered when planning a non-potable water reuse system. The major issues include water rights, economics, market and demand, storage considerations, water quality and design criteria and operation.

Water rights

The legal right to reuse water must be held by the municipality or sanitation district (water rights are typically administered by state agencies) proposing to develop a water reuse system. In Colorado, for example, reusable water rights exist only for water imported from another basin or from deep groundwater. Since the imported water was not available for historical use by others, it can then legally be reused by the agency that imported the water. Other states generally have more liberal requirements for reuse of wastewater effluent.

Economics

An economic evaluation should be done comparing the cost of reclaimed water to an alternate supply, including surface water or groundwater. In some cases, the alternate supply may be purchased water from a neighbor.

The comparison of alternatives should be based on similar criteria. Reclaimed water is essentially drought proof. It is available at essentially constant rates regardless of hydrologic conditions. Thus, the availability of a firm yield must be included in the comparison. Further, the evaluation should include costs for treatment and delivery of water to the point of use for each alternative.

Very stringent effluent quality requirements may make the cost of treatment for discharge greater than reuse. The difference in cost between discharge and reuse should be considered in the economic evaluation of water reuse.

Market and demand

Water used for urban irrigation is a significant portion of the total demand experienced in municipal systems. Irrigation uses include residential customers with small areas of turf, to large individual sites such as golf courses, parks and greenbelts. Due to the relative expense of a dual distribution system to serve residential customers, non-potable reuse systems for urban irrigation typically serve a limited number of large customers with significant water demand.

In assessing the feasibility of serving a specific site, the location and magnitude of irrigation water demand relative to the source (wastewater treatment plant effluent), must be considered. Large candidate irrigation sites within the service area should be identified, and a preliminary estimate of the water demand made. Costs of reclaimed water transmission can be estimated and a unit cost (dollars per 1,000 gallons) to serve a specific site developed.

Irrigation rates are used to establish water demands for non-potable water reuse systems. The amount of water used varies, depending upon the use of the turf and practices of irrigation managers. Generally, golf courses require more water than parks and greenbelts. Also, professional irrigation managers normally use data related to evapotranspiration, which is the total water loss from the soil, including that by direct evaporation and that from plants' surfaces, giving off moisture, to apply only the amount of water required by the vegetation. To the extent available, site-specific data of water requirements should be used to establish reclaimed water demand.

Peaking factors (peak-day to average-day water demand) are used to size reclaimed water system components. Due to the relatively small number of customers normally served by reclaimed water systems, and the practice of limiting irrigation to times when people are not present, peaking factors are higher than for potable water systems. Peak-day demand for reclaimed water can be 6 to 7 times greater than average-day demands.

Storage considerations

Water storage is a major consideration in reuse systems. Two types of storage are normally considered — seasonal storage and daily storage. Seasonal storage is used to store reclaimed water produced during non-irrigation seasons. The benefit of seasonal storage is that it significantly increases the yield of the reclaimed water system.

Daily storage is used to help with the operation of the reclaimed water system. Daily storage allows the reclaimed water treatment plant to operate relatively continuously throughout the day, filling the storage during low demand periods. During high demand periods, the stored water supplements the water produced by the treatment plant. Daily storage can be located within the distribution system, or at the individual irrigation sites. For instance, water features at golf courses can be used as daily storage. At a minimum, daily storage should hold 18 hours of the maximum treatment production. Care must be used in the design of open storage to reduce the algae growth that occurs in storage ponds and to minimize the aesthetic effects of variable water levels during the fill and draw operation.

Water quality

The quality of reclaimed water has implications for both the public health as well as system operation. Approximately 20 states have adopted requirements for water reuse systems. There are no U.S. Environmental Protection Agency (EPA) standards for water reuse, and the requirements of the state regulatory agencies vary. Several states relate types of reclaimed water use, or potential public exposure, to reclaimed water quality. For instance, parks and ball fields may be considered to have unrestricted access by the public, requiring higher water quality than a golf course or cemetery, which is considered to have restricted access and thus less potential exposure for the public.

Several states control reclaimed water quality by regulating concentrations of contaminants and also by requiring a minimum level of treatment. Controlled contaminants include microbes and suspended solids.

The nutrient content of reclaimed water is an issue to consider in planning reuse systems. Phosphorus and nitrogen can be a benefit for landscape irrigation, and may be considered in marketing the water. However, potential problems with runoff and groundwater protection need to be considered in operation and management of the systems.

Other water quality considerations are total dissolved solids (TDS), bicarbonate content and sodium levels in the water. These components of reclaimed water can have an impact on irrigation practices and also on sensitive vegetation. The abundance of sodium without calcium or magnesium in irrigation water can cause "plugging" of the soil, thus reducing the water's infiltration rate. Also, aesthetic characteristics of reclaimed water such as color and odor may be an issue, depending upon the end use of the reclaimed water.

Design criteria

Diurnal flow patterns. Reuse systems depend upon wastewater as their raw (source) water. In evaluating system components, availability of raw water must be considered. Daily wastewater flow variations can be significant. Typical low flow conditions in wastewater treatment plants occur in the early morning, when reclaimed water demand is high. Storage in the reclaimed water system can be used to supplement supply when raw water supply is inadequate.

Treatment processes. Reuse of water for unrestricted access generally requires secondary effluent to be filtered and disinfected. Filtration rates used in the design of reuse systems are similar to those used for potable water systems, typically ranging between 3 to 5 gallons per square foot. Coagulants typically include alum, ferric chloride and sodium aluminate.

Disinfection is typically accomplished by chlorination or ultraviolet irradiation (UV). Requirements for chlorine contact vary between states. For instance, California requires 120 minutes of modal detention for chlorine contact. UV effectively disinfects the wastewater and avoids the hazards associated with chlorine.

Reliability. Reuse systems may be less reliable than potable water systems. The reclaimed water use and marketing issues should be taken into consideration when choosing a system.

Solids disposal. The amount of solids generated in reclaimed water treatment plants is a function of raw water (secondary effluent) total suspended solids (TSS), contribution by coagulant, and the effectiveness of TSS removal. Options for disposal are site specific, including land disposal, landfill and return to the sewer.

Distribution. System design criteria include color-coding or marking of the pipes to clearly differentiate the reclaimed and potable water systems. Purple is the accepted color for reclaimed water pipes. Generally, horizontal and vertical separation of reclaimed water pipes from potable water pipes is required. Cross connection protection devices need to be incorporated into the system to protect the potable system from inadvertent contamination by the reclaimed water.

Public acceptance

Informed consumers are absolutely essential to obtaining support for the development of a reclaimed water system. Successful water reuse programs have well-developed public relations and public involvement programs. The specific mechanism used for obtaining public acceptance is extremely important. An example would be public meetings.

The reuse system can be used to inform the public about water conservation, water treatment, wastewater treatment and water resources throughout the design, construction and operating phases of the project. Demonstration plots using a wide variety of turfs and other landscaping materials can be established in parks as well as at the plant, to demonstrate water conserving plant materials and the effectiveness of reclaimed water. Literature targeted at specific age groups, interests and professions should be prepared. Landscape architects, irrigation system designers, horticulturists, golf course designers, park managers, plumbers, public health officials and many other professions will be interested in understanding technical and public health aspects of the projects. Their support can help assure acceptance of the project by the public at large.

Case studies

The city of Aurora is the third largest in Colorado, with a population of 270,000. The city has been recycling water since 1968. A 2.5 million gallons per day plant built in 1981 is now being expanded. Water recycling is an important part of the city's water resource plan, and has proven to be an economical means of extending available water resources. The new $15 million system will be capable of producing 5 million gallons of water per day to irrigate parks, golf courses and greenbelts. Also under consideration is an even larger system, which will include reservoirs to store reclaimed water produced during non-irrigation seasons for use during the summer.

Aurora uses 50,000 acre-feet of water each year. With the new expansion, 1,200 acre-feet of water will be recycled each year. With the addition of storage and transmission pipelines, this system is projected to meet a demand of 5,000 acre-feet of recycled water by 2020.

Another example of innovative water reuse was developed by the Inverness Water and Sanitation District which serves a business park where 16,000 people work. The district is located in Englewood, Colo., south of Denver. The district faced increasing water demands and decreasing water supplies. Groundwater used by the district is limited, as the water level of the aquifer in some locations has dropped more than 200 feet in the last five years. Also, increasingly stringent water quality regulations severely impact wastewater treatment and disposal requirements.

A new 1.2 million gallon per day water recycling system was planned, designed and constructed. The district now completely recycles all of its water. The system uses an innovative filtration system designed to remove 97 percent of the phosphorous from the recycled water used for irrigation of an 18-hole golf course and surrounding business park. The cost of recycled water for irrigation is 60 percent less than the cost of alternative water supplies. By recycling water rather than pursuing other water sources, the district was able to cost-effectively transform a disposal problem into a water supply solution.

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This article appeared in the March 2000 issue of Environmental Protection magazine, Vol. 11, No. 3, p. 60.

This article originally appeared in the 03/01/2000 issue of Environmental Protection.